In the past few decades, the automotive industry has striven to expand both the number and types of vehicular functions and systems subject to computer control. Due in part to the proliferation of such computer control, however, available physical space within a vehicle has correspondingly diminished, thereby resulting in a demand for more compact control systems. At the same time, owing both to the competitive climate within the industry and to the crucial nature of some of the vehicular functions under computer control, the overall reliability of such control systems has risen to the level of paramount importance.
As an example of one such system subject to computer control, a modern automotive ignition system typically includes an ignition coil and a coil current switching device responsive to an ignition, or "drive", signal to energize the ignition coil. Some type of control circuitry, responsive to microprocessor control, provides a drive signal to the coil current switching device to thereby energize the primary side of the ignition coil.
Typical prior art automotive ignition systems have incorporated the control circuitry and coil current switching device into a single ignition module using a so-called hybrid electronics technology. Essentially, hybrid electronics is an amalgamation of integrated circuit technology and discrete electronic component technology arranged and surface mounted on a ceramic substrate.
Hybrid ignition modules have been well received in the automotive industry, but they suffer from several inherent drawbacks. First, due simply to the number and size of discrete and integrated components required for operation, the overall size of an ignition module can be quite large as compared to typical packaged integrated circuits. This problem is compounded by limitations inherent in hybrid processing technology, such as large conductor line widths and conductor routing limitations. The size and number of componentry further adds to the overall weight of the module which, as the number of such vehicular control systems increases, can become a significant factor in system design. Second, such hybrid modules are typically expensive to produce, particularly when compared to processing costs associated with comparably complex integrated circuits. Finally, because of the number of module components and interconnections therebetween, module reliability can be significantly less than that of comparably complex integrated circuits.
Designers of automotive ignition modules have attempted to address the foregoing drawbacks inherent in hybrid technology by designing so-called "single chip" ignition coil control circuits. Such circuits incorporate the control circuitry and coil driver device into a single high voltage integrated circuit, typically formed of silicon. Although typical single chip approaches address many of the drawbacks associated with hybrid ignition modules, they have their own inherent drawbacks to consider.
First, any single chip circuit incorporating a coil driver device therein must necessarily utilize a very high voltage semiconductor process for the control circuitry as well as the coil driver device. As such, this constitutes wasteful utilization of advanced semiconductor processing techniques since ignition control circuitry can generally be more cost effectively implemented with conventional integrated circuit processes.
Second, most single chip approaches provide the coil driver device in the form of a power transistor requiring drive currents on the order of 100 milliamperes. Since most automotive computer-controlled systems operate from low current regulated power supplies, the drive current demands of such single chip devices may require either more robust or supplemental power supplies for successful operation. In either case, most of the prior art single chip ignition coil control circuits further require some type of discrete componentry to limit power, supply current.
Third, prior art single chip ignition coil control circuits are somewhat limited in their range of application. Since different ignition systems incorporate different ignition coils, a wide variety of coil current requirements result therefrom. Single chip ignition coil control circuits must thus undergo expensive re-design and re-layout for each substantially higher or lower current capability version.
Finally, most prior art single chip ignition control circuits incorporate a bipolar junction power transistor as the coil driver device. Such a coil driver device is susceptible to becoming biased under a reverse battery condition so as to provide a high current flow back through the power transistor and the ignition coil, thereby resulting in potential damage to each. Such bipolar junction power transistors are further susceptible to a condition known as thermal "runaway", which occurs during high temperature and/or high power operating conditions. Essentially, thermal runaway may occur in a bipolar junction transistor because the collector current in such a device increases with increased junction temperature. This phenomena becomes a concern when driving inductive loads such that power dissipation increases with increased collector current, thereby resulting in further increased junction temperatures. This effect can become cumulative if proper corrective actions are not taken, such as through proper heat sinking and robust transistor design, eventually resulting in destruction of the transistor.
What is therefore needed is an ignition coil driver arrangement that overcomes the foregoing undesirable characteristics associated with both of the prior art approaches. Such an ignition coil driver arrangement should be easily and inexpensively produced to form an advantageously compact device. Ideally, such an ignition coil driver arrangement should include multiple channels for driving corresponding ignition coils in a multi-coil ignition control system. Such a multi-channel ignition coil driver module could be even more cost-effectively manufactured than a single channel ignition coil driver if the control circuitry is designed so that much of the circuitry is shared among the multiple channels.